Grouted Post-Tensioned Rock Anchor Assessment

This report presents the results of an extensive study of grouted post-tensioned rock anchors installed in concrete gravity dams. The purpose of the study is to obtain a better understanding of the condition and capacity of existing post-tensioned rock anchors for which grout serves as the only form of corrosion protection. The performance of grout-protected anchors of various designs under actual site conditions is assessed to determine the effects of the aging process.

Grout encapsulation was the primary means of anchor installation from the mid-1960s through to the mid-1980s. Thousands of post-tensioned steel cable anchors have been installed throughout the world using this method. Contemporary anchor installation involves the addition of plastic sheathing.

In certain cases, steel cable anchors are embedded in concrete dams to improve structural strength and stability. Such anchors also reduce the amount of concrete needed to construct new dams and enable aging dams to meet contemporary safety standards. However, over time, steel cable anchors may deteriorate; this process may reduce their load capacity or even cause them to break.

In many early installations, two-stage, post-tensioned, fully grouted anchors have been employed, for which the grout is the sole defense against corrosion. Under current standards, these anchor types would be regarded as temporary structural components. Several studies performed by experts and industry leaders around the world have detected corrosion on permanent anchors installed using the two-stage, grout-encapsulation method. These studies were reviewed for this report, and their corrosion findings lead to questions regarding the reliable service life of such anchor types.

This study investigates the reliability of grout-protected anchors by obtaining real-world data, obtained from anchors removed from dams slated for decommissioning and from redundant anchors extracted from select dams. A series of laboratory and in situ tests were performed on concrete anchors from four separate dams. This report describes the original purpose of the anchors for each dam, the physical environment, other conditions to which the anchors were exposed to during their in-service life, the condition of the anchors, as well as the methods used to sample/test the anchors and the results thereof.